The present invention relates to a method and apparatus for measuring shear stress and in particular, the invention relates to an improved shear stress transducer for use with viscous or viscoelastic fluids.
Shear stress transducers that measure the shear stress exerted on a solid surface by a viscous or viscoelastic fluid are known in the art and thus, reference may be had to U.S. Pat. No. 4,464,928 issued Aug. 14, 1984 to John M. Dealy which teaches a method and associated apparatus for the measurement of shear stress. A method and apparatus for the measurement of rheological properties involving the use of a shear stress transducer is the subject of U.S. Pat. No. 4,571,989 issued Feb. 25, 1986 to John M. Dealy.
In one of the embodiments described in U.S. Pat. No. 4,464,928 the shear stress is determined by measuring the deflection of an elastic beam, where the beam is rigidly fixed at one end with the shear sensitive surface, which we will call the active face, forming tee opposite end. This is a cantilever configuration in which the lateral load (the shear force to be measured) is applied to one end while the other is fixed. This embodiment has the advantages that it is simple to manufacture and robust However, since the maximum deflection of the beam occurs at the active face, in order to optimize the sensitivity of the device it is necessary to measure the deflection as closely as possible to this end.
This may interfere with its installation in a rheometer, flow channel or processing machine. Furthermore, if the fluid of interest is under pressure, it will flow into the shear stress transducer. One way of preventing this, which was mentioned in the teaching of the aforementioned earlier patents, is to install an elastomeric seal around the active face of the beam. This can be a premolded seal, similar to an "O" ring, or a cured-in-place elastomer such as a silicone rubber. In either case, however, there will be a limit on the pressure that can be withstood without dislodging the seal or otherwise interfering with the measurement of the shear stress. This limit may be as low as 2,000 psi. Since industrial processes often involve pressures considerably higher than this, this limits the application of the shear stress transducer in industrial manufacturing operations.